The invention generally relates to equipment used to deposit initially plastic concrete that hardens to form slabs for floors, road surfaces, etc. The term xe2x80x9cplasticxe2x80x9d in this context refers to concrete that can be poured and shaped, but that will not easily flow or level itself under the force of gravity when pooled as does a true liquid. Concrete is plastic from the time of mixing and for a period thereafter depending on the type and amount of cement powder used, additives that speed or retard the hardening, and the temperature of the plastic concrete.
To use plastic concrete as a building material to construct rigid concrete slabs and other configurations that form floors, decks, roadways, runways, and bridges, the concrete must first be placed, then leveled, and finally screeded to create the final surface geometry and elevation. xe2x80x9cPlacingxe2x80x9d is the initial deposition of the plastic concrete. xe2x80x9cLevelingxe2x80x9d is the removal, addition, and shifting of placed concrete to create nearly the desired geometry or profile and elevation of the top surface. xe2x80x9cScreedingxe2x80x9d is a final step performed after leveling that provides the final desired profile and elevation, gives the top surface a smooth texture, compacts the plastic concrete, and removes remaining voids that may affect strength or durability. Screeding is performed by a flat-surfaced screed that is passed across the plastic concrete. Frequently the screed is vibrated during use to compact and remove voids from the plastic concrete.
To construct a concrete floor in a warehouse for instance, the first step is generally to erect forms of a suitable material at the perimeter of the intended area. Next a subsurface of gravel, compacted sand, or other particulate material is deposited and leveled. Frequently, reinforcing bars or mesh is placed above the subsurface but below the intended concrete surface to provide tensile strength for the hardened concrete.
The concrete deposition begins with placing the plastic concrete inside the forms. The process of placing concrete for a project is accomplished in one or more ways. Plastic concrete may be discharged directly from the chute of a concrete mixing truck. It of course may also be mixed at the site. In any case, the mixed, plastic concrete is conveyed to the desired area of the subsurface by means such as wheelbarrow, motorized concrete buggy, or concrete bucket suspended by a crane or forklift over the subsurface. Plastic concrete may also be pumped to the desired location with specialized concrete pumps.
No matter which of these traditional means of placing concrete is used however, the operator of the particular placing means employed controls where and how much concrete is placed. Since the operator is generally proceeding without a precise visual or other reference point showing the amount of concrete required and the amount placed, the predictable result is that the initial elevation and profile of the placed concrete is only a very rough approximation of the desired final elevation and profile.
The next step is leveling, which redistributes the placed, plastic concrete to a close approximation of the desired final distribution and profile. High spots are knocked down and low spots are filled in. Excess concrete is removed and insufficient amounts supplemented. Workers using shovels, rakes, and concrete xe2x80x98come-alongsxe2x80x99 frequently perform the leveling. Alternatively, mechanical means may be employed for this redistribution, including plows, augers, oscillating beams and the like.
The last step of forming the concrete mass is screeding. The screed is moved across the surface of still-plastic concrete to conform the concrete""s exposed vertical-facing surface to the desired final profile and elevation. To accomplish this, the screed itself must be precisely controlled as to its elevation, either by riding on carefully set forms or by a continuously and automatically adjusted screed control means responding to an external reference signal, such as a laser beam, GPS signal, etc.
A variety of screed means are commonly employed, including straight beams, trusses, and rollers in single or multiple configurations. Screed means frequently vibrate or oscillate to further smooth and consolidate the concrete surface.
The need during leveling to redistribute or shift concrete after it has been placed and before it can be screeded is a major source of inefficiency in the overall process of concrete flatwork construction. Costs are increased. Delays are incurred. Quality, as reflected by measures of floor flatness and floor level (FF/FL) may suffer, if the redistribution is not accurately completed. And the ultimate strength and durability of the hardened concrete may also be affected.
The invention aims to improve the efficiency of traditional means of placing, leveling, and screeding concrete by reducing or eliminating the need to redistribute and shift concrete during the leveling step and then further, by integrating the screeding with the leveling. It accomplishes these ends by using a machine that automates and combines at least the placing and leveling activities. The screeding activity can also easily be included in a preferred embodiment of the invention.
The machine has a placing element that relatively evenly distributes the plastic concrete along an advancing deposition front. The machine includes a leveling element integrated with the placing element. In this machine, leveling occurs immediately after placing in a way that creates an approximate profile and height of the concrete and assures an adequate amount of placed concrete across the deposition front. Excess placed concrete is shifted to subsurface areas not yet having any placed concrete in a way that provides a reasonably accurate elevation and profile for the leveled concrete.
My machine preferably also includes a screeding element. Screeding preferably occurs in an integral step that immediately follows leveling and may be referenced to any convenient surface elevation and geometry control using conventional means. Screeding may even be done manually.
This machine makes possible a process for forming a concrete mass comprising the first step of depositing a first strip of concrete sequentially along a first predetermined path and for a predetermined distance. Then almost immediately a second strip of concrete is deposited immediately adjacent to the first strip of concrete along a second predetermined path and for a predetermined distance. This process then continues depositing of strips of concrete in this manner for a predetermined number of iterations until the desired mass of concrete has been formed. The process forms an advancing front of plastic concrete that advances strip by strip and transversely to the predetermined paths until the entire mass of concrete has been deposited and leveled. Of course, the predetermined paths need not be linear, but can be any desired shape or configuration. However, in many cases the predetermined paths will be straight and approximately parallel to each other.